Continuous thermoprecipitator



Feb. 8, 1955 R. M. STRONG ET AL 2,701,467

CONTINUOUS THERMOPRECIPITATOR Filed Feb. 19, 1952 2 Sheets-Sheet l w ne an Emma:

LIGIT 79 RHEOS e INVENTORS ROBERT M. STRONG JOSEPH BURWELL FICKLENIII United States Patent CONTINUOUS THERMOPRECIPITATOR Robert M. Strong and Joseph Burwell Ficklen III, Pasadena, Calif.

Application February 19, 1952, Serial No. 272,376

Claims. (Cl. 73-48) The present invention relates to a device and method for separating and collecting any and all kinds of particulate matter suspended in gases or vapors for the purpose of studying purity of such atmosphere or to obtain for industrial, scientific or other uses, gas or vapor which is free from such particles.

An object of the present invention is to provide an improved device using the principle of two surfaces of different temperatures to separate and collect particles suspended in gas or vapor; to provide a movable mat upon which said particles may be deposited, means for varying the temperature of said heated surface; to provide a device wherein any and all kinds of particles may be removed from said gas, and means for regulating the rate of travel of said movable mat.

Another object of the present invention is to provide a device adapted to continuously collect particles suspended in gas or vapor for the purpose of study or to obtain for industrial or scientific use, a gas free from all of said particles, by drawing said gas or vapor through an angularly disposed narrow passage, having side walls which may be parallel or diverge outwardly or converge inwardly, and which terminates adjacent a heated element, spaced above a movable cold collecting surface, adapted to collect said particles which are caused to deposit thereon under the action of said heated element, and means for varying the temperature of said heated element dependent upon the kind of particles suspended in said gas, and including means adapted to move said cold collecting surface back and forth under said heated element.

Still another object of the present invention is to provide an improved device capable of continuously collecting particles suspended in any gaseous atmosphere, as for example in the area of brass foundries containing lead and zinc fumes, mine atmosphere containing silica and other particles, also atmospheres containing mixtures of r silica and iron oxide, radioactive isotopes and the like, by drawing said gaseous atmosphere through an angularly disposed narrow passage, which terminates adjacent a narrow element adapted to be heated by any suitable means and which is spaced above a cold collecting surface adapted to collect said particles as a result of said gaseous atmosphere passing over said narrow heated element, a means for regulating the rate at which said gas is drawn over said narrow heated surface, and including means associated with an exhaust chamber to permit visual examination of the air stream to determine if any suspended particles are escaping past said cold collecting surface.

A further object of the present invention is to provide an improved device for carrying out the above-described method, and which is simple in construction, and one which may be portable so that it can be readily moved to different places for the purpose of sampling the atmosphere which it is desirable to study or may be constructed as a permanent structure capable of removing any an all kinds of particles suspended in the atmosphere to thereby obtain a particle free atmosphere for industrial, scientific or medical use.

An important feature of the present invention is predicated on the fact that regardless of the kind of heating element or means for producing the desired heat, a two fold phenomenon exists. First, if a body is warmed to a temperature slightly above that of the air or gas which may be passed over it, there is produced immediately adjacent to the heated surface of the body a zone which is entirely free from any suspended particles which may 10 of the particulate recorder in accordance with the present "ice be in the air or gas. Second, in this connection, it is also known that no such particle free zone exists adjacent to the surface of a body which is slightly cooler than the surrounding gas or air. When these two phenomena are properly oriented and controlled with respect to each other, they form a means for the removal and collection of the suspended particles in the air or gas.

The invention will be described further in detail with reference to the accompanying drawings in which:

Fig. 1 is a perspective view showing the preferred form invention.

Fig. 2 is a sectional view of the removable cover and the intake unit.

Fig. 3 is a sectional view of the removable cover and intake unit taken substantially on line 33 of Fig. 2.

Fig. 4 is a sectional view of the carrying case and the thermoprecipitator unit mounted therein, and

Fig. 5 is a top plan view of the instrument with the carrying case and precipitator unit covers removed.

Referring now to Figs. 1 to 4 the invention comprises a carrying case generally indicated as 10, having a removable cover 11, and a removable front panel 12. Upon this panel 12, are mounted a plurality of controls for initiating an air sampling cycle presently to be described. A pair of handles 13 are mounted on the removable cover 11 to facilitate in carrying the instrument.

As shown in Figs. 4 and 5, the case 10 has enclosed therein a thermoprecipitator unit generally indicated as 14. This unit comprises a body 15 having a rectangular open chamber 16. The upper peripheral edge of the body 15 is provided with'a channel 17, adapted to receive a small flexible rubber tube 18 which extends above the upper edge of the chamber 16. The body 15 is provided with four threaded external bosses 19, which are adapted to receive four knurled thumb screws 20 extending through apertures provided in a removable cover 22. It will be noted in Fig. 4, that the inner edge of the channel 17 is slightly lower than the outer edge, to insure that when the thumb screws 20 are drawn down, the removable cover 22 will properly compress the rubber tube 18 to make the chamber 16 an airtight chamber.

The left-hand end wall of the chamber 16, is provided with spaced apertures 25 which lead into an exhaust manifold 26. The right-hand end wall of the chamber 16 is provided with a sealing type sleeve bearing 27. A movable block generally indicated as 28 is mounted on the inside of the chamber 16. This block 28 is provided with a threaded opening 29 which is adapted to receive a threaded shaft 30. This shaft 30 is threaded the full depth of the opening 29 in the block 28 when the block 28 is in the position shown in Figs. 4 and 5, and also extends through the sleeve bearing 27 in the righthand wall of the chamber 16.

The mechanism for rotating the shaft 30 to thereby move the block 28 laterally in the direction of the arrow in the chamber 16 is shown in Figs. 4 and 5. This mechanism comprises a motor 31 adapted to make one revolution per hour which is mounted on the bottom of the carrying case 10. The motor 31 drives two gears 32 and 33 having a 1:1 ratio. The gear 32 is mounted on the shaft of the motor 31, and is in driving engagement with the gear 33. The gear 33 is secured to a sleeve 34, which is keyed to the threaded shaft 30 and is slidably mounted thereon. The sleeve 34 is provided with a shoulder 37 to facilitate in positioning the gear 33 when in driving engagement with the gear 32.. The outer end of the sleeve 34 extends through the wall of the case 10 and is threaded to receive a threaded member 38. Further, the outer end of the shaft 34) is threaded to receive a manual control knob 4d. in the present instance, the inner end of the shaft 30 and also the block 28 may be threaded and tapped for twenty-four threads per inch. Since the motor makes one revolution per hour and the gears 32 and 33 have a lzl ratio, it will be evident that the block 28 will be moved laterally in the chamber 16 only one and one-quarter inches in thirty hours.

Whenever it is desirable to move the block 28 intermittentiy by manual means rather than continuously by the motor 31, it will be only necessary to backoif themember 38 sufiiciently far to permit the sleeve 34 and therewith the gear 33 to be moved inwardly out of driving engagement with the gear 32. Thereafter, the block 28 may be moved intermittently to any desired position in the chamber 16 by manually turning the knob 40 and shaft 30. Although the block 28 is described to be moved lengthwise in the chamber 16, it would be possible to construct the device to move the block 28 crosswise producing a satisfactory result.

An intake unit generally indicated at 44, is mounted in any suitable manner on the upper face of the removable cover 26. This unit comprises a body 45, including an intake tube 46 which is in communication with an intake chamber 47. The body 45 is provided with a shoulder portion 48 adapted to receive a removable cap 49. This cap 49 is provided with a plurality of small apertures 59 positioned near the center of the side wall of the cap 49. When the cap 49 is mounted on the shoulder portion 48 of the body 45, the apertures 56 will be below the upper open end of the intake tube 46. With this arrangement there is provided what may be called a settling chamber 51. When air is drawn inwardly through the apertures 50 in the cap 49, it must travel upwardly to enter the intake tube 46. In the present device, particles larger than approximately microns settle to the bottom of the settling chamber 51, to be kept out of the instrument. However, the small suspended particles are drawn into the instrument. It may be desirable to sample air from remotely located areas. This may be accomplished by removing the removable cap 49, and attaching a rubber tube to the upper end of the intake tube 46, with its free end leading to the location where the desired tests are to be made.

Referring to Figs. 2 and 3, the intake chamber 47 is in communication with an angularly disposed narrow diverging passage 53, which terminates in a narrow rectangular slot 54 on the lower face of the removable cover 22. This rectangular slot 54 intersects the edge of a milled channel 55 in which there is mounted in any suitable manner, an insulating strip 57. The insulating strip 57 is provided with an angularly disposed milled notch 58. The notch 58 matches the rectangular slot 54 and the angular diverging passage 53. It will be evident that by arranging the diverging passage 53 and notch 58 in an angular relation relative to the lower face of the removable cover 22, when air is drawn into the instrument, in a manner presently to be described, the air will be directed over a wide area.

To facilitate in precipitating suspended particles normally present in atmosphere to be tested, it is necessary to provide a suitable element for heating the air. This may be accomplished, for example, by mounting a heating element 60, in the form of a #27 Nichrome resistance wire drawn across the full length of the notch 58, with its free end threaded through small apertures extending from each end of the notch 58. The ends of the heating element 60 are connected to suitable terminals 61 securely mounted in the insulating strip 57 and extending downwardly from the cover 22. The wire 60 may be held under tension by connecting one end thereof to a suitable spring loaded stud 61, Fig. 3. These terminals 61 are adapted to engage suitable contacts 62 mounted on the outside of the body connected by means of leads 63 to the power supply. It is important to state that, although a single element 60 is shown to be drawn across the notch 58, a plurality of small wires may be drawn parallel with the notch 58 or a plurality of special wires may run parallel with the direction of flow of the air through the notch 58. Further, the heating element used may be angularly disposed across the notch 53. It is equally well to mention that although a resistance wire has been described as the heating element, it is possible to construct an instrument used for industrial purposes, where the heating element may comprise a suitable metal strip heated by any suitable means, as for example, hot air, water or steam to mention a few possible means for producing a heated surface.

One form of a cool collecting surface for carrying out the precipitating function of the present invention is shown in Figs. 2, 3, and 5. This cold surface comprises, for example, a 2 x 3 glass microslide 65, shown in solid and dotted lines, adapted to stand extremely rough treatment in cleaning and handling, or the cold collecting surface may be a thin microcover slip 48 mm. X

72 mm. if the record is desired for oil immersion work. These slides 65 are positioned on the upper surface of the movable block 28, by means of eight small pins 66 arranged to engage the corner portions of the slide 65, Fig. 4. The collecting surface on the slides 65 may be opaque, transparent, translucent, wet, moist, sticky, dry, plain or coated. Instead of using a single large slide, it (would be possible to use a plurality of spaced small sli es.

An insulating spacer 68 is mounted on the lower face of the removable cover 22, adjacent the edge of the notch 58 and heating element 60. This spacer 63 maintains a fixed distance of a few thousandths of an inch between the heated element and the cold collecting surface on the slide 65, when the removable cover 22 is clamped in place. Thus, there is formed a narrow passage through which the heated atmosphere being sarnpled must pass. It is generally known when air passes over a heated element, the suspended particles move away from the heated element 66 to be precipitated 0n the cold surface. By drawing the air through an angularly disposed passage, the collected particles will tend to feather out as they are spread over the width of the collecting slide 65, rather than being impinged onto the collecting surface. By precipitatively collecting the particles, they are not broken up but remain in their natural form. The collecting surface of the slide may intersect the dust free area around the heated element 60, or it may be placed on the edge of the dust free area, or even outside but adjacent to this area where partial removal of particulate matter is desired.

Returning now to the passage of the air through the instrument. After the air flows over the microslide 65 it is drawn through the apertures 25 into the exhaust manifold 26. An eyepiece 70 is mounted on the upper surface of the manifold 26. A light source incorporated in a conventional tyndallmeter 71 is mounted on the right-hand wall of the manifold 26, at an angle so that it will send an intense light beam across the air stream inside the manifold 26 at the focal point of the eyepiece 70. In this way it is possible to visually examine the efiiuent air streams to determine if any of the suspended particles are escaping past the cold collecting surface on the slide 65. If any particles are observed in the air stream, it will be evident that the temperature of the heated element 60 is too low or the rate at which the air is sucked through the instrument is high. A suitable transparent insert 71' has also been provided in the cover 22 Fig. l, to examine the deposit on the slide 65 without having to remove said cover 22 while the device is in operation.

The air is drawn out of the manifold 26 through a rubber tube 72 and travels through a conventional flow meter 73 mounted on the front of a removable front panel 12. From the flow meter 73 the air passes through a rubber tube 74 a moisture trap 75 and out through a needle valve 76 mounted on the inside of the case 16, Fig. 4, and out through a suction tube '77, connected to any suitable suction means not shown. The rate at which the air moves, may be regulated by adjusting the needle valve 76 through a control knob 7 6.

The current to the heating element 6%) may be controlled by a knob 79 connected to a rheostat not shown, and may be indicated on an ammeter 86, Fig. 1. In the present instrument, an ammeter reading of 2.5 amperes for 20 ccm. per minute sampling rate is required.

A plurality of toggle switches 81 and panel light 82 are mounted on the front of the panel 12, are labeled as shown in Fig. l, for controlling the motor 31 the heating element 60 and the light source 71, respectively.

In operation, after the assembled unit shown in Fig. 1 has been placed in the area where the air is to be sampled, the switch to the heating element 6%) is turned. Thereafter, the current is adjusted to the value of 2.5 amperes by means of the rheostat, not shown, as will be indicated on the ammeter 80. This reading, in the pres ent invention, has been established for a 20 ccm. per minute sampling rate. After the element 69 been heated, the switch controlling the motor 31 is turned on, and the block 28 starts to move laterally in the cham ber 16. Simultaneously the switch controlling the tyndallmeter 71 is turned on and suction of the air through the instrument is iniitated.

As the polluted air travels inwardly through the apertures 50 in the cap 49 the particles larger than approximately microns, settle to. the bottom of the settling chamber 51 with the smaller suspended particles trav' elling upwardly to enter the intake tube 46. As, the air continues to travel downwardly through the intake chamber 47 to enter the angularly disposed narrow diverging passage 53, and out through the rectangular slot 54, in the lower surface of the removable cover 19, it is caused to spread out. When the air leaves the rectangular slot 54, it travels over the heated element 60 which is warmed to a temperature slightly above that of the air being tested. This differenial between the temperature of the element 60 and the air being tested, causes the suspended particles to move away fromthe heated element 60 to produce a dust free area immediately around the heated element 60. In the present instrument, the cold surface on the microslide 65 which is mounted on and moving with the block 28, intersects the dust free area previously mentioned. Since the temperature of the microslide 65 is slightly cooler than the air being tested, there is no such particle free area around the surface 65. Thus, if these two conditions are properly oriented and controlled with respect to each other, the suspended particles are precipitately collected on the cool surface of the slide 65.

After the air leaves the collecting surface 65, it is drawn through the apertures 25, into the exhaust manifold 26. The air passing through the manifold passes an intense light beam from the tyndallmeter light 71. This light illuminates the area around the particles which may be escaping past the collecting surface 65, and this condition may be observed through the eyepiece '70. From the manifold 26 the air is drawn through the rubber tube 72 into and through the flow meter 73 on out through the needle valve 76, and exhausted from the suction means not shown.

It is to be understood that the above detailed description is furnished to illustrate by way of example one suitable manner of carrying our invention into effect and the invention is not to be regarded as. restricted inscope to the details set out in the said description, but covers broadly apparatus constructed in the principles set out above.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim 1. In apparatus for continuously determining the amount of foreign material in a gaseous fluid such as air: means defining a narrow passage through which said gaseous fluid is adapted to flow, said passage diverging outwardly toward its discharge end, said end terminating in a narrow elongated discharge slot substantially rectangular in outline; an elongated heating element extending lengthwise of said discharge slot for heating said gaseous fluid as it flows from said passage tocreate a zone about said heating element that is entirely free from particulate matter; means having a collecting surface cooler than the warmed gas, said surface being inclined at an angle relative to said passage and being at least at the outer edge of said free zone so as to receive a deposit of the foreign particles in said gaseous fluid; power actuated means for automatically and continuously moving the means having the collecting surface relative to the discharge slot of said passage; and means downstream of the collection area of said collecting surface for determining the freedom of the gaseous fluid from said foreign particles.

2. ln apparatus for determining the amount of foreign material in a gaseous fluid: means defining a narrow passage through which said gaseous fluid is adapted to flow, said passage diverging outwardly toward its discharge end, said end terminating in a narrow elongated discharge slot substantially rectangular in outline; an elongated heating element extending lengthwise of said discharge slot for heating said gaseous fluid as it flows from said passage and to thus create a zone about said element that is free from particulate matter; means having a collecting surface cooler than the warmed gaseous fluid, said surface being inclined relative to said passage and being substantially at the outer edge of said free zone so as to receive a deposit of the foreign particles in said gaseous fluid; manual means for moving said means having the collecting surface relative to the discharge end of said passage; and means downstream of the collecting surface for determining the freedom of the gaseous fluid from said foreign particles.

3. In apparatus for continuously determining the amount offoreign material in a gaseous fluid: means defining a narrow passage through which said gaseous fluid is adapted to flow, said passage having side walls which taper toward its discharge end, said end terminating in a narrow elongated discharge opening; and elongated heating element extending lengthwise of said discharge slot for heating said gaseous fluid, as it flows from said discharge opening for heating said gaseous fluid as it leaves said passage to create a zone about said heating element that is free from particulate matter; means having a collecting surface cooler than the warmed gaseous fluid, said surface being inclined relative to said passage and being located to substantially touch at least the outer edge of said free zone so as to collect a deposit of the foreign particles in said gaseous fluid; means for effecting relative movement between said collecting surface and the discharge end of said passage; and means downstream of said collecting surface for determining the freedom of said gaseous fluid from said foreign particles.

4. In apparatus of the class described: means defining a narrow passage through which gaseous fluid is adapted to flow, said passage diverging outwardly toward its discharge end, said end terminating in a narrow, elongated discharge slot substantially rectangular in outline; an elongated heating element extending lengthwise of said dis charge slot for heating said gaseous fluid as it flows from said passage so as to create a zone about said heating element that is free from foreign particles that may be in said gaseous fluid; transparent means having a collecting surface, said means being cooler than the warmed gaseous fluid and inclined at an angle relative to the fluid: stream discharged from said slot, the collecting surface of said means at least touching the periphery of the particle free zone; and power actuated means for continuously moving the transparent means relative to the discharge end of said passage.

5'. In apparatus of the class: described: means defining a narrow passage through which gaseous fluid is adapted to flow, said passage diverging outwardly toward its discharge end which terminates in a narrow, elongated discharge slot; a plurality of elongated heating elements extending lengthwise of said slot for creating a particle free zone by warming the gaseous fluid flowing across said element; means having a collecting surface intersecting at least the peripheral edge of said zone at an inclined angle relative to said passage, said means being cooler than the warmed gaseous fluid; and means for effecting relative movement between said means having said collecting surface and the discharge end of said passage.

6. In apparatus for determining the amount of foreign particles in a gaseous fluid: means defining a narrow passage through which said gaseous fluid is adapted to flow, said passage terminating at its discharge end in a narrow, substantially rectangular discharge slot; an electrical heating element at said discharge end, said element being adapted to be warmed to a higher tempera ture than the gaseous fluid in said passage so as to create a zone about said element that is free from particulate matter; means having a collecting surface cooler than the warmed gaseous fluid, said surface being angularly disposed with respect to the stream of gaseous fluid, flowing from said discharge slot and being at least at the outer edge of said zone; and means downstream of said discharge slot for determining the freedom of the gaseous fluid from foreign particles after said fluid has passed said collecting surface.

7. In apparatus of the class described: means defining a narrow passage through which gaseous fluid is adapted to flow, said passage diverging outwardly toward its discharge end which terminates in a narrow, elongated discharge opening; an elongated heating element extending lengthwise of said opening for creating a particle free zone by warming the gaseous fluid flowing across said element; and means having a collecting surface intersecting at least the peripheral edge of said zone at an inclination relative to the fluid stream leaving said passage, said surface being cooler than the fluid contacting it.

8. In apparatus for determining the amount of foreign particles in a gaseous fluid: means defining a narrow passage through which said gaseous fluid is adapted to flow, said passage terminating at its discharge end in a narrow, substantially rectangular discharge slot; an elongated heating element at said discharge end, said element being adapted to be warmed to a higher temperature than the gaseous fluid flowing thereover so as to create a zone about said element that is free from particulate matter; means having a collecting surface cooler than the warmed gaseous fluid, said surface being substantially parallel with the stream of gaseous fluid flowing from said slot and being at least at the outer edge of said zone; means forming a chamber for the means having the collecting surface; an exhaust manifold having a plurality of connections with said chamber for receiving gaseous fluid therefrom; and tyndallmeter means for determining whether particles are in the gaseous fluid which has passed the collecting surface.

9. In apparatus of the class described: means defining a narrow passage through which gaseous fluid is adapted to flow, said passage having an inlet and an outlet and which terminates in a narrow, elongated discharge opening; means defining a precipitation chamber connected with the inlet of said passage, said precipitation chamber being adapted to collect larger particles of foreign matter in said gaseous fluid; an elongated heating element extending lengthwise of said discharge opening for creating a particle free zone by warming the gaseous fluid flowing across said element; and means having a collecting surface intersecting at least the peripheral edge of said zone, said means being cooler than the warmed gaseous fluid.

10. The invention defined by claim 9 wherein the means defining said precipitation chamber includes side walls having a plurality of peripherally spaced inlet openings and the connection between said chamber and the inlet of said passage includes a tubular part within said chamber, said part having an inlet end spaced longitudinally of the plane of the inlet openings in said side walls and located adjacent to the outer end wall of said chamber.

11. The invention defined by claim .9 wherein the connection between the precipitation chamber and passage inlet includes a tubular member having an inlet opening at the outer end thereof; and the means defining said chamber comprises a cylindrical wall and an end wall closing one end of the cylinder formed by the cylindrical wall, the other end of said cylinder being removably secured to a wall member of the apparatus, said cylinder being of larger inside diameter than the outside diameter of the tubular member.

12. In apparatus of the class described: means defining a narrow passage through which a gaseous fluid is adapted to flow, said passage diverging outwardly and terminating at its discharge end in a narrow, elongated discharge slot; a heating element at said discharge end, said element being adapted to be warmed to a higher temperature than that of the gaseous fluid in said passage so as to create a zone about said element that is free from particulate matter; means having a cool collecting surface inclined relative to the stream of gaseous fluid flowing from said discharge slot and being in contact with said zone; means downstream of said discharge slot for determining whether foreign particles remain in said fluid after same has passed the collecting surface; and means for controlling the flow of fluid through the apparatus.

13. The invention defined by claim 12 wherein there is means for regulating the temperature of the heating element.

14. The invention defined by claim 12 including a fluid flow meter for measuring the fluid flow through the apparatus.

15. In apparatus of the class described: means defining a narrow passage through which gaseous fluid is adapted to flow, said passage diverging outwardly and terminating at its discharge end in a narrow, elongated slot; a heating element at said discharge end, said element being adapted to be warmed to a higher temperature than that of the gaseous fluid in said passage so as to create a zone about said element that is free from foreign particles; means having a collecting surface cooler than the warmed gaseous fluid and inclined relative to the stream of gaseous fluid flowing from said discharge slot, said surface being closely associated with said particle free zone so as to collect a deposit of foreign particles entrained in said fluid; means downstream of said discharge slot for determining whether foreign particles remain in said fluid after the same has passed the collecting surface; and means for controlling the temperature of said heating element.

16. The invention defined by claim 15 wherein the heating element is an electrically heated element; and there is means for measuring the current supplied to said heating element.

17. In the method of continuously determining the amount of particulate pollution in a gaseous fluid, the steps of: forming a wide, relatively thin stream of said gaseous fluid; at one point warming said stream throughout its entire width to thereby create an elongated zone entirely free from pollution particles; collecting a deposit of said particles on a collecting surface cooler than the warmed stream, located at least at the peripheral edge of said zone and inclined relative to said stream; effecting movement of said collecting surface relative to said free zone, said movement being continuous in one direction; determining whether there are any residual particles in said stream after said stream has encountered said collecting surface; measuring the rate of fluid flow of said stream; and correlating the warming of the gaseous stream with the rate of flow thereof so that substantially all of the pollution particles will be deposited on said collecting surface.

18. In the method of continuously determining the amount of particulate pollution in a gaseous fluid, the steps of: forming a wide, relatively thin stream of said gaseous fluid; at one point warming said stream throughout its width to thereby create an elongated zone entirely free from pollution particles; collecting a deposit of said particles on a collecting surface cooler than the warmed stream, located at least at the peripheral edge of said zone and inclined relative to said stream; effecting movement of said collecting surface relative to said free zone, said movement being continuous in one direction; determining whether there are residual particles in said stream after said stream has encountered said collecting surface; and correlating the warming of the gaseous stream with the rate of flow thereof so that substantially all of the pollution particles will be deposited on said collecting surface.

19. In a method of the class described: directing a wide, relatively thin stream of gaseous fluid onto a collecting surface at an inclination to said surface; warming said stream across the width thereof at a location closely adjacent to the point of contact thereof with said surface while maintaining said surface at a cooler value than the gaseous fluid at the initial point of contact with said surface; determining whether there are any residual particles in said stream downstream of said initial point of contact; and correlating the warming of the gaseous stream with the rate of flow thereof so that substantially all of the pollution particles will be deposited on said collecting surface.

20. In a method of the class described: directing a wide, relatively thin stream of gaseous fluid onto a contacting surface at an angle to said surface; interposing a warming element in said stream across the width thereof at a location spaced from but closely adjacent to the point of contact thereof with said surface and maintaining said element at a higher temperature than the gaseous fluid so that the latter is maintained at a higher temperature than said surface at the initial point of contact of the fluid stream with said surface, and correlating the warming of the gaseous stream with the rate of flow thereof.

References Cited in the file of this patent FOREIGN PATENTS Germany July 19, 1951 OTHER REFERENCES Size of Smallest Particles Determined in Impinger Dust- Counting Methods, Brown ct al. Bureau of Mines Report of Investigations 4-802, July 1951.

Physical Methods for the Estimation of the Dust Hazard in Industry, with Special Reference to the Occupation 

